Complement receptor C5aR1 blockade reprograms tumor-associated macrophages and synergizes with anti-PD-1 therapy in gastric cancer

In gastric cancer (GC), the therapeutic response of immune checkpoint blockade (ICB) remains suboptimal. Targeting myeloid cell checkpoints might be feasible as adjuvant to current ICB regimens. We sought to evaluate the crucial role of C5aR1(+) TAMs in regulating antitumor immunity and the efficacy of combinatorial treatment with antiprogrammed cell death protein-1 (PD-1) and C5aR1 blockade. Here, we found that C5aR1 was predominantly expressed on macrophages and high level of C5aR1(+) TAMs infiltration could predict poor prognosis and inferior chemotherapeutic response. The flow cytometry (FCM) and single-cell RNA-seq (scRNA-seq) data revealed that C5aR1(+) TAMs exhibited immunosuppressive property which might contribute to CD8(+) T cell dysfunction. Blockade of C5aR1 could diminish the immunosuppressive function of TAMs and led to reinvigorated CD8(+) T cells mediated antitumor immunity. Moreover, using in vitro intervention experiment based on fresh GC surgical specimens, we discovered that C5aR1 blockade exert a synergistic effect when combined with PD-1 inhibitor for tumor clearance. Our study demonstrated that C5aR1 is a critical myeloid checkpoint and plays a crucial role in regulating the immunosuppressive property of TAMs and CD8(+) T cell immune tolerance. C5aR1 blockade reprograms TAMs and reinvigorated the cytotoxicity of CD8(+) T cells, thus improving the efficacy of anti-PD-1 therapy for tumor eradication in GC.